1.13.T-03 - Bridging In Vivo and In Vitro Evidence From New Approach Methodologies (NAMs): Comparing XETA and ToxCast Data To Identify Thyroid-Active Substances

Abstract

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The identification of thyroid-active chemicals remains a major challenge for hazard assessment. The thyroid system can be affected through various mechanisms, including hormone synthesis, metabolism, and receptor-mediated pathways. To improve mechanistic understanding, it is essential to establish the coherence between in vivo and in vitro New Approach Methodologies (NAMs) targeting the thyroid axis.
The Xenopus Eleutheroembryonic Thyroid Assay (XETA) is an OECD test guideline (OECD TG 248) for identifying thyroid active chemicals. The XETA uses fluorescent Xenopus laevis transgenic eleutheroembryos to identify activity of the thyroid axis and how it can be altered by chemicals. As it uses non-protected embryonic life-stages, according to the current EU legislation on the protection of experimental laboratory animals, the XETA is considered a NAM.
In the present work, we conducted XETAs with reference chemicals selected with existing knowledge on their specific mechanism of action in mammals to expand the dataset of reference chemicals that were used in the OECD validation of the test guideline. In total, 60 reference chemicals have now been tested in the XETA. These substances were selected to represent a broad range of mechanisms, including thyroid peroxidase inhibition, thyroid hormone synthesis disruption, altered deiodinase activity, and competitive binding to thyroid hormone transport proteins.
To further explore the mechanistic concordance between in vivo and in vitro NAMs, we have interrogated the CompTox Chemicals Dashboard containing relevant high- throughput bioactivity data (ToxCast andTox21 databases) to retrieve assay data for the same set of chemicals. We have then performed a comparative analysis to evaluate the level of agreement between the responses in the XETA and the high-throughput in vitro mechanistic signatures.
This combined analysis aims to (i) assess the reliability of the XETA in capturing thyroid-related mechanisms of action; (ii) identify consistencies and discrepancies with in vitro assay outcomes; and (iii) contribute to the integration of mechanistic data in a weight-of-evidence approach for endocrine disruptor identification. Ultimately, this work will help clarify how amphibian-based embryo assays and large-scale in vitro screening platforms can be used to support the implementation of NAMs and provide confidence in the assessment of thyroid-active substances within regulatory contexts.

Author(s)

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Presenter: Gregory Lemkine, Watchfrog S.A.,

Authors:
David Du Pasquier Watchfrog S.A. France
Barbara Duchesne Watchfrog S.A France
James Wheeler Corteva Agriscience UK Limited. Netherlands
Paula Garcia Corteva Agriscience United Kingdom
Katherine Coady Bayer CropScience United States
Laurent Lagadic Bayer AG, Crop Science Division, Environmental Safety Germany
Gregory Lemkine Watchfrog S.A. France